The distance between optical terminals of optical fibre transmission systems is limited by the optical power that can be launched into optical fibre by optical transmitters of the optical terminals, the loss and dispersion of optical fibre interconnecting the optical terminals, and the sensitivity of optical receivers of the optical terminals. Where the distance between desired end points of an optical fibre transmission system exceeds the maximum distance between optical terminals, optoelectronic repeaters have been provided. Each optoelectronic repeater comprises an optical receiver for converting the optical signal to an electrical signal, electronics for regenerating the electrical signal, and an optical transmitter for converting the regenerated electrical signal to an optical signal for transmission to the next optoelectronic repeater or to a terminal of the system. There are two main techniques for multiplexing signals in such systems, which operate by wavelength division or time division.
In Wavelength Division Multiplexed (WDM) optical fibre transmission systems which use optoelectronic repeaters, the optical signals are optically demultiplexed at each repeater, so that the signal at each distinct wavelength is coupled to a respective optical receiver for conversion to a respective electrical signal, each respective signal is applied to a respective optical transmitter operating at a distinct wavelength, and the transmitted signals are optically multiplexed for transmission to the next optoelectronic repeater or to a terminal of the system.
As the line rates of optical fibre transmission systems increase into the 2.5 Gbps to 10 Gbps range, higher speed electronics are needed in optoelectronic repeaters, and this increases the cost of optoelectronic repeaters.
Optical amplifiers, for example Erbium Doped Fibre Amplifiers (EDFAs), amplify optical signals directly without converting them to electrical signals. Because EDFAs do not require high speed regeneration electronics, they can be cheaper than optoelectronic repeaters for high speed optical fibre transmission systems.
Moreover, in WDM optical fibre transmission systems, the EDFAs can amplify optical signals at multiple wavelengths without optically demultiplexing them, thereby avoiding the costs of optical multiplexing and demultiplexing, and the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. Consequently, EDFAs can also be cheaper than optoelectronic repeaters for WDM systems. However, degradation by noise and dispersion effects builds up when optical amplifiers are used. Thus a regenerator may be necessary after several optical amplifier stages, to rebuild the data signal and remove the noise and dispersion degradation.
Disregarding intermodal dispersion which only occurs in multimode fibre (not used in practice for high capacity systems), dispersion, also known as Group Velocity Dispersion, in fibre at least, occurs as a result of two mechanisms:
1 intramodal dispersion--within a single mode different frequencies travel along the fibre at different speeds; PA0 2 material dispersion--the phase velocity of plane waves in glass varies with frequency.
Dispersion is the derivative of the time delay of the optical path with respect to wavelength. The effect of dispersion is measured in picoseconds arrival time spread per nanometer `line width` per kilometer length (ps nm.sup.-1 km.sup.-1). The magnitude of intramodal and material dispersions both vary with wavelength, and at some frequencies the two effects act in opposite senses. It is generally possible, on a given single mode fibre, to find a wavelength around which there is negligible dispersion, or, conversely, to design a fibre to have minimum dispersion at a desired wavelength. References to dispersion herein will mean the sum total of group velocity dispersion effects.
Dispersion in optical fibre presents serious problems when using light sources whose spectrum is non-ideal for example broad or multispectral-line, or when high data rates are required, e.g. over 2 GB/s. This problem has previously been addressed, at least partially, in four ways. Firstly, by operating at or close to the optical frequency at which the dispersion is a minimum, for example at a wavelength of 1.3 micron in conventional silica fibre. The frequency does not generally correspond with the frequency of minimum transmission loss and attempts to modify the fibre to shift its frequency of minimum dispersion usually result in some loss penalty. This solution has limitations for two reason. Firstly manufacturing variations will always occur. Secondly, a non linearity called four wave mixing seriously degrades WDM signals near the dispersion zero of one piece of fibre. Accordingly, it may be preferable to operate in a given region of dispersion which may not include the dispersion zero.
The second way of overcoming the problem is to use a source with a near ideal narrow linewidth spectrum. The limits for improvement in this respect have been reached since at higher bit rates, the Kerr effect becomes significant. This is where the index of refraction varies with intensity, which causes self phase modulation, or cross phase modulation. The resulting frequency redistribution means that dispersive degradation increases again.
Thirdly, dispersion compensators have been used to equalise the dispersion with an element of equal and opposite dispersion. Such dispersion compensators may take the form of length of fibre, a Mach Zehnder interferometer, an optical resonator, or a Bragg reflector. Some of these compensators can give a variable, controllable amount of compensation.
A fourth technique is to change the modulation at the transmitting end. On example is discussed in EP-A-0643 497. Dispersion produces an FM to AM conversion effect which can facilitate bit detection and thereby extend transmission distance without controlling or compensating dispersion. The dispersion causes shifting of adjacent signal components of different wavelengths, resulting in either energy voids or energy overlaps at the bit transitions. Constructive interference in an overlap causes a positive peak in the optical signal, while a void produces a negative peak. These positive and negative peaks represent an AM signal which may be detected to reproduce the original bit stream.
The document proposes the additional step of adjusting the output power of one or more of the inline amplifiers to further stabilise the dispersion-induced optical signal energy voids and overlaps and thereby further improve the detection thereof. This method requires difficult precision engineering and so is impractical for commercial exploitation.
With the different types of dispersion-shifted fibre, dispersion compensating fibre, and dispersion-compensating filters that could make up a given link, determining the dispersion of a link is no longer the simple operation of multiplying the length in km by the 17 ps/nm/km dispersion characteristic of standard single mode fibre. Moreover, when there are optical switches or controllable optical dispersion compensators in the link the dispersion can change as a function of time.
There are several laboratory test instruments available that measure this dispersion, on a static basis. However, they are large, expensive and cannot be used while a signal is present at the same wavelength. Some such instruments require both ends of the fibre be at the same location, and so can only be used to test components of a system before installation. Certainly they are not suitable for incorporation into any element of a practical transmission system.
One attempt to control the effects of dispersion in a high speed transmission system is known from EP-A-0700 178, as shown in FIG. 1. Reference numeral 41 is an optical transmitter, 42 is an optical receiver, 43 is an optical fibre, 44 is a tuneable light source, 45 is a tuneable filter, 46 and 47 are optical amplifiers, 48 is an optical detector, 49 is a drive circuit, 50 is a tuneable filter, 51 is a repeater, 52 is a sweep controller, and 53 is a transmission characteristic measuring section.
The drive circuit 49 is controlled by the sweep controller 52 to sweep the emission wavelength of the tuneable light source 44. For example, when the tuneable light source 44 is constructed from the tuneable semiconductor laser, sweeping can be accomplished by varying the currents Ip and Id; in the case of a semiconductor laser of other configuration, sweeping of the emission wavelength can be accomplished by continuously varying the temperature. The optical signal with the thus swept emission wavelength is transmitted along the optical fibre 43 and via the repeaters 51, and is detected by the optical detector 48 of the optical receiver 42, where the received result is applied to the transmission characteristic measuring section 53 which measures the transmission characteristic between the optical transmitter 11 and the optical receiver 12. Based on the result of the transmission characteristic measurement, the emission wavelength of the tuneable light source 44 and the wavelength transmission characteristics of the tuneable filters 45 and 50 are so set as to achieve the best transmission characteristic.
Variable dispersion compensators (not shown) may also be controlled to find an optimum measured transmission characteristic.
The transmission characteristic measuring section 53 may be constructed to measure the transmission characteristic by measuring bit error rates (BER). Alternatively, it may be contructed to measure the transmission characteristic using an eye pattern.
Since the eye pattern opens wide when the transmission characteristic is good, the emission wavelength of the tuneable light source 44 may be adjusted so that the eye pattern opens widest. As an adjusting means in this case, control may be performed manually while observing the eye pattern, or alternatively, automatic control by means of computer processing may be employed.
An alternative way of measuring the bit-error rate is to measure the Q value (electrical SNR). The Q value is expressed using the signal level difference (=signal amplitude) between emission and no emission as the numerator and the sum of the standard deviations of noise during emission and during no emission as the denominator. When a Gaussian distribution is assumed for the noise distribution, the bit-error rate given by the Q value agrees with the minimum-value of the actually measured bit-error rate. Other methods, such as measuring the transmitted waveform and using specifications of equal bit-error rate curves, may also be employed.
However, such measurements of transmission characteristics do not give a direct measurement of dispersion, since many other factors affect the bit error rate, the eye pattern and the Q value. There is no straightforward way of eliminating the effects of these factors. Therefore, accurate values of dispersion cannot be derived by these methods outside a laboratory.
A second problem with this system is that the sweeping technique causes a degraded BER, (a figure of 10.sup.-11 is quoted). This would be unacceptable in many working systems.